venerdì 10 maggio 2013

The German V2 jammer

The German V2 jammer

The three primary transmitter units - power
oscillator, modulator, and audio oscillator

In 1944 the threat of
the German Vergeltungswaffen weapons caused a great deal of activity in
electronic countermeasures work. Initial signals collection around Peenemunde
showed some unusual emanations in the 20-70MHz area, and it was surmised that
the V2 was guided by this frequency. Subsequent analysis could detect nothing in
that band of frequencies when the weapons began falling on London and the
English countryside, further deepening the mystery. As it turned out, postwar
accounts indicated that early surveillance intercepts were most likely a
tracking system developed by a Professor Wolman, using triangulation from
multiple sites around the V2 launch site, and of course no one on the Allied
side knew it was initially guided by an on-board gyro and vanes in the exhaust
stream and then became a straight ballistic package. Later in its use, a radio
guidance system was in fact introduced to improve accuracy (Lorenz
Leitstrahlstellung), apparently working around at a frequency around
45MHz and installed on the last 25% of V-2s produced. Its effectiveness was not
completely clear, given that the guidance system only worked during the powered
portion of the missile's flight. Since there was a huge amount of Allied
electronic surveillance work being done by 1944, interception of even these
short transmissions may have had a part in the race to develop countermeasures.

At any rate, the British began deploying a ground based jammer with
about 75KW output to counter the perceived threat, and used highly directional
antennas to point toward Peenemunde. Thinking ahead, the Americans were
concerned about what happened if and when the V2 show went on the
road...the Germans had already proved to be masters of mobile weaponry.
In any event, the ARQ-11 was the initial product of a well known EW company by
the name of Airborne Instruments Laboratory (now AIL Technical Services
Operations, currently a business unit of EDO). The jamming system was quickly
built and tested in a B-24 during the summer of 1944. It used a pair of British
"micropup" radar pulse triodes in a push pull power oscillator configuration to
get 1,700 watts output at the lowest frequency point of 20MHz. The need to get
such power out of tubes designed for pulse applications required a huge
"canister vacuum cleaner" blower of the type pioneered by Electrolux at the rear
of each valve, simply to obtain any life out the tubes at all.

One of the REL-1 micropup tubes

The power oscillator was a
tuned push-pull oscillator with no further amplification, pictured below. Note
the plate current meter,which reads the combined plate current from the 6kV
supplies - 3 amperes full scale!

T-102/ARQ-11 power oscillator

It's probably a
miracle that this set has survived - it has numerous parts in it that radio
amateurs used after the war to make kilowatt rigs. Below are shown some details
of its exquisite construction.

This view shows the plate coils and swinging link. Five sets
of these coils were required to cover 20-70MHz.

The modulator is designed to provide the proper grid
excitation for the three modes of operation shown on the transmitter panel, and
included a clipper stage, a 3MHz local oscillator, a full wave rectifier, an
output keying stage, and a power supply. The 30Hz to 30,000Hz sine wave
oscillator input coming from the O-28/ARQ-11 is amplified and shaped to a square
wave, which is used to key the local oscillator. The output of that stage (a
pair of 807 tetrodes) is rectified and applied to the grids of the main 811
modulator tubes.

MD-42/ARQ-11 modulator

O-28/ARQ-11 audio oscillator

Three power supplies were used
with three phase 400~ input to get the 6kV at 2 amperes needed for the
transmitter. Only about a dozen complete ARQ-11 sets were manufactured before
the contract was terminated because of the pace of ground advance in Europe.
This particular power oscillator is serial number 9, the modulator is serial
number 6, and the audio oscillator is serial number 5. This set is missing only
the C-187/ARQ-11 junction box/power panel and the associated R-21/ARQ-11
receiver. If anyone has any documentation or pieces of this set, I would deeply
appreciate them contacting me.